A number of very useful infrared detectors have been developed which feature an extrinsic semiconductive substrate, a portion of which serves as a detector element, coupled to a charge readout structure located in an overlying layer. Devices of this nature are illustrated and described, inter alia, in a co-pending U.S. patent application (Ser. No. 614,277) and a U.S. Pat. No. 4,142,198 ("Monolithic Extrinsic Silicon Infrared Detectors with an Improved Charge Collection Structure") of the Applicants herein. Both of the aforementioned are the property of the present assignee. The above-referenced patent, which discloses an improved planar geometry with respect to the pending (and presently allowed) application, features a semiconductor substrate of a first conductivity type having an overlying, continuous epitaxial layer for the transfer of charge to a utilization circuit by means of a CCD structure located in an overlying insulating layer of SiO.sub.2 or the like. An aperture in a radiation absorptive covering layer positions and defines the individual detector within the underlying substrate. Charge is transferred from the detector element of the substrate to the epitaxial layer by, inter alia, a heavily doped buried layer of the first conductivity type disposed adjacent a portion of the interface between the substrate and the epitaxial layer and a heavily doped surface layer of the first type of conductivity formed in the epitaxial layer adjacent and overlying the substrate in the detector area. These combined, heavily doped layers, in conjunction with a heavily doped backside layer, serve as electrodes for the application of a photon-generated, carrier-depleting electric field across the substrate. The lightly doped substrate is maintained at an electrically insulative temperature (below 40.degree. Kelvin) so that the absorption of photon energy is required to raise the energy band of the majority carriers of the substrate to conduction. Absent the absorption of such energy, these carriers are immobile or "frozen out", resulting in a state of electrical insulation.
An important feature of the patented device is the provision of a heavily doped buried layer of the second conductivity type adjacent the interface of the substrate and epitaxial layer and spaced from the aforementioned buried layer of opposite conductivity type. This buried layer serves the vital function of insulating the substrate from the overlying CCD structure. As the CCD must accomplish charge transfer by the creation of potential wells attractive to the majority carriers of the substrate (minority carriers of the epitaxial layer), the absence of such a doped buried layer (which provides a re-combination site for carriers within the substrate) would introduce a certain amount of noise into the output of the device and would additionally prevent the room temperature testing of CCD devices even in a thin (3-4 micrometer) epitaxial layer.
The aforementioned device, and other devices similar in principle, are conventionally operated in two separate steps. First, the mobile carriers of the substrate, energized or "unfrozen" by absorbed radiation, are "pushed" into the charge transfer circuitry by the aforementioned electric field applied across the backside and collection electrodes. If, for example, the substrate is doped with the atoms of a Group 3 element such as gallium or indium, standard procedures would involve the application of a higher potential to the backside electrode than to the collector electrode to inject the generated mobile carriers into the charge transfer structure. The second step involves the CCD transfer of the generated mobile carriers within the epitaxial layer (in which these are now minority carriers) to the utilization circuitry.
The inventors herein have found that, operated in accordance with the above-described conventional mode, useful radiation detection may be achieved with such devices. However, device performance has been found to be seriously flawed for certain applications by the presence of undesired noise and high operating temperatures. This noise and heat are believed by the inventors herein to result from the device's P-I-N structure occasioned by the above-described geometry which incorporates interfacing layers (isolation layer, substrate (insulative at low temperatures) and backside electrode) "sandwiched" together forming a combination which is necessarily forward biased in the conventional charge collection mode. The forward biasing of this combination leads to the "double injection" of carriers into the substrate. That is, the forward bias condition results in a flow of substrate current composed, in part, of the transfer of both holes from the backside electrode and electrons from the isolation layer into the substrate. As a result, considerable current passes through the substrate. (It has been found that, under certain substrate doping conditions such as heavy gallium doping, a forward bias of 5 volts or greater can, in fact, drive a substrate to avalanche breakdown.) While a certain amount of (leakage) current is tolerable, the avalanche phenomenon, effectively creating a short circuit through the substrate, is a major hindrance to the perfection of the operation of a device of the type described. The breakdown problem may be moderated by the use of high-breakdown dopants, such as indium which has a breakdown of over 40 volts. However, gallium is particularly desirable for infrared detection. While indium and gallium both function well in the 3 to 5 micron range, (radiation penetration of clouds, etc.), indium is not an effective detector material in the other significant IR "window" existing at 8 to 12 microns. This window, which includes night vision, is critical to most tactical applications and, thus, gallium doping is essential for the reliable operation of long wavelength systems.